A wireless communication system has been widely developed to provide various kinds of communication services such as voice and data. Generally, the wireless communication system is a multiple access system that can support communication with multiple users by sharing available system resources (bandwidth, transmission power, etc.). Examples of the multiple access system include a code division multiple access (CDMA) system, a frequency division multiple access (FDMA) system, a time division multiple access (TDMA) system, an orthogonal frequency division multiple access (OFDMA) system, a single carrier frequency division multiple access (SC-FDMA) system, and a multi carrier-frequency division multiple access (MC-FDMA) system.
Device to device (D2D) communication refers to a communication scheme that a direct link is configured between user equipments (UEs) to allow the UEs to directly transmit and receive voice and data to and from each other without through a base station (evolved NodeB; eNB). D2D communication may include user equipment-to-user equipment (UE-to-UE) communication, peer-to-peer communication, etc. Also, the D2D communication scheme may be applied to machine-to-machine (M2M) communication, machine type communication (MTC), etc.
D2D communication is considered as a solution for solving load of a base station, which is caused by data traffic which is rapidly increased. For example, according to D2D communication, since data are transmitted and received between devices without a base station unlike the conventional wireless communication system, overload of a network may be reduced. Also, with the introduction of D2D communication, advantages of process reduction of the base station, power consumption reduction of devices which join in D2D communication, increase of data transmission rate, increase of network capacity, load distribution, cell coverage enlargement, etc. may be expected.